FIG. 4 shows the outline of a transfer unit of the prior art for printed circuit boards (hereafter referred to as "board") on which electronic components are mounted. In FIG. 4, a transfer motor 21, board guide 22, and driving pulley 23 are fixed to the shaft of the transfer motor 21. A pulley 25 is disposed at both ends of the board guide 22. A belt 24 is wound around the two types of pulley 23 and 25, and runs parallel to the board guide 22.
One end of a guide width adjusting screw 26 is directly connected to a guide width adjusting motor 27, and the other end is screwed to a nut 30 provided on a support frame 32 of the board guide 22. One end of a guide rail 28 having grooves lengthwise is supported by a bearing 33 provided on a support frame 29, and the other end is loosely inserted into a bearing 33 provided on a support frame 32. A board guide 34 is fixed to a support frame 29 in parallel to the board guide 22. The board guide 34 has the same belt mechanism as the board guide 22 inside, and the belt runs synchronized to the belt 24. A cylinder 35 is provided at the end of the board guide 34 as a stopper.
The operation of a board transfer unit as configured above is explained next.
Drive power is transmitted to the belt 24 through the pulley 23 when the transfer motor 21 is rotated at the constant speed. The belt 24 then runs parallel to the board guide 22 via the pulley 23. The board is transferred by placing it on the belt 24. After starting the board transfer, the cylinder 35 is actuated, and the rotation of the transfer motor 21 is stopped when the board contacts the cylinder 35, thus stopping the board at a specified position.
Next, the operation for transferring boards with different widths is explained.
When the guide width adjusting motor 27 is rotated, the guide width adjusting screw 26 directly connected to the shaft of the guide width adjusting motor 27 also rotates. By rotation of the guide width adjusting screw 26, the nut 30 which is screwed to the guide width adjusting screw 26, moves back and forth according to the rotation direction of the guide width adjusting screw 26. Therefore, the board guide 22 remains parallel to the board guide 34 while it moves to adjust the width therebetween. The above operation allows the transfer of boards with a range of widths. FIG. 5 shows a velocity diagram of the board during transfer in the prior art.
If boards are transferred at high speed using the board transfer unit of the prior art, the transfer speed of the board will suddenly increase or decrease as shown in FIG. 5. This will apply a large acceleration or deceleration force to large, small, light, and heavy components temporarily mounted on the board and held by the adhesivity of a material such as solder paste printed on the board, causing dislocation, inclination, or dropping off of mounted components.